I can try to answer this the best I can as like everyone else has said, its hard to know for sure the purity of any drug bought off the street however, to give comparison, I?m in CT and have been lucky enough in the past to be able to obtain almost pure diamorphine. It was almost always pure white and being that I was an I/V user, when mixed with water would turn a goldish brown, and have NO residue left at the bottom of the spoon after being sucked into the syringe.
A good sign that there is a lot of cut would be if there was almost as if there were sugar that didn?t dissolve at the bottom, or chunky ?gook?. Basically if there wasn?t residue, it was either good purity, or it was cut with something water soluble. There are many other variables too, the most reliable to me being taste. Once you get to know the taste of good pure H, you?ll never forget it.
As far as overdosing, the first signs to look for are eyes rolling up in the head, skin turning pale, very slowed breathing, loss of consciousness, slurred speech and just overall being out of it. I?ve saved two people I was with from overdose on H. It was scary, they had a definite look about them where I could tell something was VERY wrong. Hope this helps. If you go 20-25 mins after a line and none of these symptoms present themselves, I?d say it was safe to assume you/they are in the clear. As fast as I/V, those symptoms occur almost immediately.
-TheProfessor-